This invention relates in general to oilfield perforators and more particularly, to an energy absorbing device used in conjunction with a shaped charge perforating device to lessen physical damage to a gun body housing upon detonation of the charge.
Explosive shaped charge well perforating devices are often used in perforating well casing and the surrounding earth formations in the production of hydrocarbons. In a typical embodiment, a plurality of shaped charges are mounted in a fluid-tight, cylindrical, metal housing or on an elongated bar member which is adapted to traverse the borehole to be perforated. The shaped charges are mounted in the housing or on the bar member at longitudinally spaced intervals, with their axis of perforating directed generally laterally thereof. A more detailed description of a typical perforating apparatus is contained in U.S. Pat. No. 4,428,440, which is incorporated herein by reference.
The shaped charge most common in well perforating is a conical shaped charge. A conical shaped charge consists of an explosive material having a substantially conical cavity formed in the front face. A metal liner material covers the face of the cavity. Upon detonation the shape of the explosive cavity focuses and propagates a progressive wave front against the outside surface of the metal liner. At the pressures generated the metal acts as a fluid. Metal in fluid form is focused into a "jet" stream. The resultant focusing force moves particles to form a jet which lengthens as the wave front advances from apex to base of the conical cavity. The extreme high pressure, particle laden, jet stream breaks down and moves aside any material upon which it impinges. Penetration of such material is a result of the amount of pressure and the kinetic energy in the jet stream. One form of conical shaped charge used in well perforating is illustrated in U.S. Pat. No. 4,387,773, which is incorporated herein by reference.
Oil well shaped charges typically are enclosed in a metal charge case. The relative weight of the case provides an inertial backup for the explosive energy and serves to reflect a portion of the energy back toward the charge liner. Unfortunately, the cases that meet the weight and space requirements of the perforating gun device are not fully effective in absorbing or reflecting all of the explosive energy. The charge case under the explosive action may expand and eventually break-up. The resulting debris scatters within the gun body. While the present perforating gun bodies function to keep the debris from being deposited within the well, they tend to deform under the explosive forces of the detonation process. If these forces are excessively large it can result in excessive swelling, fracturing and/or severing of the gun body. Thus, two factors contribute to gun body damage: (1) high velocity debris from the shaped charge case impacting the gun body interior, and (2) the blast wave caused by detonation of the shaped charges.
These and other disadvantages are overcome with the present invention by providing method and apparatus for perforating a well casing and the surrounding formations using a lined shaped charge employing protection against both the explosive blast wave and charge case debris.